From the viewpoint of energy problems and environmental issues, production of a chemical substance (chemical energy), capable of being stored/transported, by light energy (for example, solar energy) is required, such as in plants. Plants use a system called the Z scheme that excites light energy in two stages. According to this constitution, plants use solar energy to obtain electrons from water (H2O) and, thus, to reduce carbon dioxide (CO2), whereby plants synthesize cellulose and sugars.
However, the technology to obtain electrons from water and decompose CO2 by an artificial photochemical reaction to produce chemical energy achieves very low efficiency. For example, Jpn. Pat. Appln. KOKAI Publication No. 2011-094194 discloses a photochemical reaction device including an oxidation reaction electrode that oxidizes H2O to produce oxygen (O2) and a reduction reaction electrode that reduces CO2 to produce carbon compounds. The oxidation reaction electrode obtains a potential to oxidize H2O by a semiconductor photocatalyst using light energy. Further, the oxidation reaction electrode oxidizes H2O to obtain electrons. On the other hand, the reduction reaction electrode obtains a potential to reduce CO2 by a semiconductor photocatalyst using light energy and the electrons obtained by the oxidation reaction electrode. According to this constitution, the reduction reaction electrode reduces CO2 to produce formic acid (HCOOH). Thus, in Jpn. Pat. Appln. KOKAI Publication No. 2011-094194, a Z-scheme type artificial photosynthesis system imitating plants is used to obtain a potential required to reduce CO2 and produce chemical energy.
However, in Jpn. Pat. Appln. KOKAI Publication No. 2011-094194, the conversion efficiency from solar energy to chemical energy is approximately 0.04% and very low. This is because the energy conversion efficiency of semiconductor photocatalysts that can be excited by visible radiation is low.
Jpn. Pat. Appln. KOKAI Publication No. 10-290017 provides a configuration in which a silicon solar cell is used to obtain the reaction potential and catalysts are provided on both sides of the silicon solar cell to produce a reaction. In S. Y. Reece, et al., Science. vol. 334. pp. 645 (2011), a configuration in which silicon solar cells are layered is used to obtain the reaction potential, and catalysts are provided on both sides of the silicon solar cells to produce an electrolytic reaction of H2O. The conversion efficiency from solar energy to chemical energy in both of those devices is 4.7%. A solar cell used herein is a triple junction solar cell, and the conversion efficiency from solar energy to electrical energy is 8.0%.
However, in a multi-junction solar cell, although the energy conversion efficiency is higher than those of an optical catalyst and a single junction solar cell, the cost also becomes high.
On the other hand, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-157801, in the electrolysis of H2O, 1.23 V is required as a theoretical voltage. Thus, it is difficult to electrolyze H2O through a one-step reaction using an optical catalyst or a single junction solar cell. Accordingly, in Jpn. Pat. Appln. KOKAI Publication No. 11-157801, a two-step reaction is utilized as in the Z-scheme type artificial photosynthesis system in Jpn. Pat. Appln. KOKAI Publication No. 2011-094194. More specifically, first, using a photocatalyst, H2O is oxidized to produce O2 and protons (H+) and, at the same time, reduce trivalent iron ion (Fe3+) to divalent iron ion (Fe2+). Next, using an electrolysis system by a usual external power supply, Fe2+ is returned to Fe3+ and, at the same time, H+ is reduced to produce hydrogen (H2). Namely, when an oxidant (Fe2+) of a redox medium is produced using a low-cost photocatalyst, H2 can be produced while reducing an electrolysis voltage.
However, in this method, it is difficult to collect O2. When solar light is applied, an oxidation reaction of H2O and a reduction reaction of Fe3+ forcibly occur due to a photocatalyst. Thus, after the reduction reaction of Fe3+ has reached saturation, an H2 producing reaction is required to forcibly occur. On the other hand, after the oxidation reaction of Fe2+ has reached saturation, Fe2+ as a reductant disappears, meaning H2 cannot be produced. Although solar light irradiation is required to produce Fe2+, the adjustment is difficult because the solar light irradiation depends on weather conditions. Namely, it is difficult to obtain chemical energy, such as O2 and H2, according to demand.
As described above, there is required a device which efficiently converts solar energy into chemical energy according to demand while reducing cost.